1. Field of the Invention
The present invention relates to an automobile exhaust system for use in an automotive vehicle such as, for example, a motorcycle for substantially purifying exhaust gases emitted from a combustion engine of the automotive vehicle engine.
2. Description of the Prior Art
The Japanese Laid-open Patent Publication No. 08-312324, published Nov. 26, 1996, discloses a prior art exhaust system for a motorcycle combustion engine, of a kind to which the present invention pertains. The prior art exhaust system disclosed therein includes a generally cylindrical or tubular outer shell having a conical end cap secured to a downstream open end of the outer shell with respect to the direction of flow of exhaust gases from the combustion engine, and a generally cylindrical or tubular inner tubular shell disposed within the tubular outer shell in a fashion substantially coaxial therewith for the passage of the exhaust gases. A sound absorbing material such as, for example, glass wool having a multiplicity of interstices or open celled pores defined therein is filled in an annular chamber, which is delimited between the tubular outer and inner shell, so that the exhaust gases introduced from the combustion engine into the tubular inner shell can be subsequently introduced into the annular chamber between the tubular outer and inner shells through a plurality of communicating perforations defined in a circumferential wall of the tubular inner shell. The exhaust gases so introduced into the annular chamber flow through the interstices in the sound absorbing material with the consequence that exhaust noises can be attenuated.
This known exhaust system also includes a tailpipe having a diameter smaller than that of the tubular inner shell, which is disposed within the conical end cap in a fashion coaxially with the conical end cap so that the exhaust gases flowing through the tubular inner shell can be guided into the tailpipe to facilitate a further silencing of the exhaust noises.
Thus, the prior art exhaust system of a type discussed above makes use of two exhaust sound silencing means; a means of guiding the exhaust gases, introduced from the combustion engine into the tubular inner shell, into the sound absorbing material through the plural communicating perforations defined in the entire periphery of the tubular inner shell and a means of employing a reduced diameter of either the tubular inner shell or the tailpipe. However, it has been found that the use of those two means is still insufficient for an intended sound silencing effect. The sound silencing effect may be enhanced if the cross-sectional area of the tailpipe is reduced, but this may result in increase of the resistance to flow of the exhaust gases, which in turn results in reduction of the exhaust efficiency and hence, reduction in output of the combustion engine.